The present invention relates to ceramic composites, more particularly mica-containing ceramic matrix composite materials which are reinforced with inorganic fibers.
Ceramic fiber reinforced ceramic matrix composites consist of ceramic fibers supported in a matrix of ceramic material. Inorganic whiskers and fibers have heretofore been used to reinforce glass, glass-ceramic, and ceramic material. A further objective of fiber and whisker reinforcement in glass, ceramic and glass-ceramic materials is to increase the toughness of the material. To accomplish this, it is desirable that the fibers in such materials debond relatively easily from the matrix phase at the fiber/matrix interface. For example, in some desirable composite materials, the energy needed to cause a fracture at the fiber/composite interface is only a fraction of the energy needed to fracture the fiber. These materials typically exhibit crack deflection and fiber pullout properties which serve to greatly increase the toughness of the composite body. Such weak mechanical coupling of the fibers to the ceramic matrix has been accomplished using several methods. For example, U.S. Pat. Nos. 4,935,387 and 4,984,758 describe applying sol-gel coatings to inorganic fibers. The sol-gel coatings convert, during processing, of the composite, to mica materials. The mica-coated fibers are then incorporated into a composite material. This results in a mica interlayer, between the fiber and the matrix material, which imparts tough fracture behavior to the resultant composite material. However, this method does not provide a network of mica in the matrix phase, and therefore does not provide crack deflection in the matrix phase, which would also enhance matrix fracture toughness.
U.S. Pat. No. 5,132,253 discloses a method for synthesizing alkaline earth metal containing ceramic materials by sol-gel processing, and using these sol-gel materials to coat fibers, such as silicon carbide, in fiber reinforced ceramic composite materials.
Unfortunately, the fiber coating methods described in the patents above are generally expensive and not efficient for coating large numbers of fibers at a time. As a consequence, the thickness of the interlayer coatings on the fibers is typically non-uniform. In addition, many of the fibers become bridged, meaning that two fibers become joined together. During further processing that takes place during the manufacturing process, these bridges often break, leaving unexposed fiber sections. The end result is that, the fibers are uncoated in these regions, and there is no sheet silicate interlayer between the matrix particles and the fibers. In addition, these methods do not provide a network of mica in the matrix phase, and therefore can not provide crack deflection in the matrix phase which would otherwise further enhance matrix fracture toughness.
U.S. Pat. No. 5,132,256 discloses a method and composition in which a ceramic matrix composite article comprises reinforcing fibers such as silicon carbide fibers disposed within a ceramic matrix, characterized in that the ceramic matrix contains dispersed mica crystallites in a proportion ranging up to but not exceeding about 20% by weight. Mica powder particles and ceramic matrix powder particles of approximately equal size are mixed together. This powder mixture is then prepregged with a fiber reinforcement material. This results in a dispersion of mica crystallites, within the ceramic matrix, which provide sites for crack deflection in the matrix. This results in improved high temperature flexural strength of the composite because ingress of oxidizing environments to the fiber/matrix interlayer is minimized. However, because the mica is added as discrete particles of 10 microns in size, they remain in domains of relatively the same size after consolidation. As a result, there is no possibility of developing a continuous, extended mica network throughout the matrix. More importantly, the amount of mica present at the fiber/matrix interface is proportional to the volume fraction of mica. Consequently, this method typically results in no more than about 20% of the interface between the fiber and matrix being comprised of mica.
It would therefore be desirable to form a more composite interface comprised primarily as a mica phase by introducing the mica into the matrix in the form of a nearly continuous network throughout the matrix. Such a continuous network would promote increased strength via fiber debonding through the mica interfacial layer and subsequent pullout, rather than crack deflection in the matrix or other sheet silicate. In addition, to further promote fiber debonding through the mica and fiber pullout, it would be desirable to achieve a more uniform and continuous fiber/coating interlayer which is inherently oxidation resistant.